Random access is a fundamental procedure for a UE, sometimes referred to as a mobile radio terminal, to have access to a wireless network. The UE may perform a random access procedure upon occurrences of the following exemplary situations: when the UE performs an initial connection with the radio network node (e.g., a base station (BS)) if there is no connection (e.g., a radio resource control (RRC) connection); when the UE first accesses a target cell in a handover procedure; when requested by a command from the BS; and a recovery procedure when there is a radio link failure or handover failure.
A UE in an idle state monitors system information broadcasted by BSs within range to inform itself about “candidate” base stations in the service area. When a UE needs access to services from a radio access network, it sends a request over a random access channel (RACH) via a suitable BS, typically a base station with the most favorable radio conditions. Because the uplink propagation conditions are usually only approximately known, the UE gradually increases its transmission power over the RACH until either the BS acknowledges the RACH preamble or a predetermined number of unsuccessful access attempts has been reached. But assuming the UE is admitted the access, a radio communications connection or link via the most suitable BS is initiated towards the UE if there are available radio resources. Uplink coverage by the BS is thus a necessity for successful random access.
In a wireless network, such as a Code Division Multiple Access (CDMA) network, uplink radio resources in coverage of a cell are limited by rise over thermal (RoT) that the cell can tolerate. The RoT is a total received power at the BS divided by the thermal noise in the cell, and the cell coverage is limited by a maximum RoT. A high RoT facilitates high uplink data rate but limits the uplink coverage, which makes it difficult or even impossible for UEs to successfully complete random access from some parts of the cell coverage area, since the RACH preamble may not be detected by a BS.
The maximum RoT, sometimes referred to as the target RoT, is either determined based on coverage requirements and/or uplink power control stability requirements. When only one UE is transmitting over an uplink connection in the cell, power control stability is minor issue because the uplink interference is likely to be dominated by the power generated by this UE. In this situation, a higher maximum RoT may be used to allow a higher signal-to-interference ratio (SINR), which enables higher uplink data rates. When multiple UEs transmit simultaneously, their SINRs will be adversely affected due to inter-UE interference. In this case, a relatively lower maximum RoT is more suitable. However, setting a lower maximum RoT can guarantee the uplink coverage, but the uplink data rate will be punished.
Accordingly, in uplink of a wireless system, there is a trade-off between the cell coverage and enabled peak transmission rates over a radio interface. This trade-off is further emphasized with enhanced uplink, which supports higher data rates than ordinary dedicated channels. An existing solution is proposed towards this problem where high RoT is allowed in some time periods during which high uplink data rate can be achieved, correspondingly uplink coverage can only be guaranteed in part of the cell serving area and random access may be failed outside of that area. In some other time periods a lower RoT is allowed during which missed random access request (preamble detection) due to insufficient SINR can be avoided over the whole cell serving area. Specifically, with respect to a low target RoT, the missed preamble detection due to out of coverage (insufficient SINR) will be decreased, but there may be more RACH access failure due to RACH access collision. On the other hand, with respect to a high target RoT, there may be some RACH access failure due to insufficient coverage. A failed RACH access due to insufficient coverage will very likely fail again if it is still carried in a time period with insufficient coverage, which both consumes additional uplink load and increases RACH access delay.
In view of the foregoing problems, it would be desirable to obtain a better trade-off between cell coverage and uplink data rate for improving random access performance in the wireless network.